This invention relates to a method of impregnating styrene/maleic anhydride copolymer particles with a blowing agent in aqueous suspension.
The impregnation of polystyrene particles with blowing agent in aqueous suspension is a well known process. Certain styrene copolymers, however, are susceptible to absorbing water from the aqueous suspending medium and retaining said water to the detriment of subsequent expanding and molding properties.
Thus, U.S. Pat. No. 3,039,977 to Ingram relates that brominated polystyrene retains water. The problem was eliminated by the addition of water-soluble metal salts to the suspension medium.
Similarly, U.S. Pat. No. 4,430,450 relates that certain pelletized emulsion-prepared styrene copolymers with alpha-alkylstyrenes, acrylonitrile, acrylates and methacrylates tend to absorb up to 3% moisture from the suspension impregnation process. Again, use of a water-soluble salt reduced the water absorption by these copolymers from 3% to less than 1%.
Neither of these references suggested that water absorptions as high as 10-12% could be eliminated by the use of salts.